In the manufacture of infrared radiation absorbing soda-lime-silica glass compositions for use in making glass articles for automotive and architectural applications, iron has been used. The iron is in the form of ferrous ions and ferric ions in the glass made under ordinary melting conditions.
In Janakirama-Rao U.S. Pat. No. 3,779,733 glass articles are formed from soda-lime-silica glass compositions in which at least 80 percent of the total iron in the glass is in the ferrous state. The patent states that the glass articles have a light transmission of greater than 70 percent at 0.25 inch thickness between wavelengths of 350 to 500 millimicrons (blue light). Both iron and tin are incorporated in the soda-lime-silica glasses, the glass composition of Example 1 being as follows:
______________________________________ Component Percent by Weight ______________________________________ SiO.sub.2 71.0 Na.sub.2 O 13.5 CaO 8.5 MgO 3.8 Al.sub.2 O.sub.3 0.2 SnO.sub.2 2.5 Iron 0.5 SO.sub.3.sup.-- 0.2 Cl.sup.- 0.1 ______________________________________
The iron content was reported as follows:
______________________________________ Component Percent by Weight ______________________________________ Fe.sub.2 O.sub.3 0.01 FeO 0.21 ______________________________________
The U.S. Pat. No. 3,779,733 further states that at least a part of the tin used be in the form of metallic tin to provide stannous ions upon the melting of the glass batch, the molar ratio of stannous ions to ferrous ions in the glass article being at least about 1:1. The stannous ions are said to act as an internal reducing agent.
The above described glass having ferrous ions does have infrared radiation absorbing properties, having a strong absorption band centered at 1060 nanometers. However, in order to obtain sufficient infrared absorption, high iron levels must be used which in turn undesirably decreases Illuminant A values below about 70 percent, which, for example, is too low to meet the Federal Illuminant A requirements of at least about 70 percent for automotive windshield glass. As is known, the Illuminant A value includes transmittance on the red end as well as the blue end of the spectrum.
The above described U.S. Pat. No. 3,779,733 glass can generally pass the Illuminant C test, which concerns transmittance in the blue end of the spectrum.